main window: hide/show top level widgets when navigating

- removes the effects of a widget's constraints when it is present
  in the navstack, but not the current index

- all widgets in the navstack now have a QWidget with a grid layout
  as the top level

- regulates the size of the main window

Change-Id: I6b0726d9777be1b33fdf164ce9dd3db5ec917ec1